Don't even bother. It is entirely abstract and bears no relationship to any actual unit structure or organization whatsoever. Plus unit sizes change over time. A U.S. regiment in 1850 would be about a thousand guys. A century later, 3000. And they are not consistent from country to country. In Great Britain, a regiment was not even a tactical unit, it was a collection of battalions that could be spread out around the globe (due to the peculiar battalion system that evolved in the British army).
If you feel you have to assign something, what you do is take the size of your army in people and divide it by the number of military units you have. That will give you a person to unit ratio.
Here is an incredibly rough and, I reiterate, totally irrelevant and arbitrary scale that you can then assign:
100-300 guys per unit: each unit is a company
1,000 guys per unit: each unit is a battalion
3,000 guys per unit: each unit is a regiment
12,000 to 20,000 guys per unit: each unit is a division
40 to 80k guys per unit: each unit is a corps
Over 80k guys per unit: each unit is an army
That's based (very roughly) on typical mid-20th century ratios and units